Dr. Dw Riche Preller—Crystalline Rocks, N. Piémont. 351 
The great diorite belt begins near Castellamonte (408 m.) in the 
Orco Valley, about midway between Ivrea and Lanzo, crosses the 
Dora Baltea at Ivrea (267 m.), Montalto and Bio, continues north-east 
to Donato, Ceresito, Graglia, Pollone, and Biella (410 m.), traverses 
the Cervo, Valley at Andorno (600m.), and thence extends to Val 
Sesia between Varallo (450 m.), Scopa, and Scopello (650 m.). From 
Donato, close to the great eastern moraine wall or Serra d’Andrate, 
the diorite is flanked on the left by serpentine and porphyrite belts, 
each about 1 kilometre in width, which extend for about 20 kilometres 
to the Sessera Valley, midway between the Cervo and Sesia Valleys. 
In all these isolated and continuous masses of basic and acid 
eruptive rocks the structure is prevalently massive and granitoid, 
rarely schistose, with essentially primitive elements without 
alteration by metamorphism. Peridotite and lherzolite are much 
more in evidence than the altered product serpentine. The granite 
with white and pink felspar, very little mica, but rich in quartz, is 
made up of smaller elements than ordinary, granite. Both it and 
the syenite masses of Traversella and Biella are closely related to 
the diorite which at Pollone, near Biella, becomes micaceous, then 
quartzose, and then passes into granite. The greenish-grey granitoid 
diorite with hornblende and biotite, often garnetiferous, is essentially 
massive and unaltered; only in contact with peridotite or gabbro is 
there an occasional tendency to prasinitic alteration by decomposition, 
but without contact metamorphism. The dioritic and porphyrite 
belts, although running parallel for 20 kilometres, are nowhere in 
direct contact, but always separated by a band of serpentine, mica- 
schist, or sericitic, quartziferous schist. The porphyrite (Gastaldi’s 
melaphyre) contains occasionally, e.g. near the Oropa Sanctuary above 
Biella, lenticular blocks of mica-schist, the former thus having 
probably consolidated somewhat later than the latter. The sericitic, 
quartziferous schist is of the same greywacke type as that flanking 
the diorite belt of Gran Nomenon and of Val Cogne in the Aosta 
Valley. Between those two diorite belts, linked by the intermediate 
diorite mass of Locana in the Orco Valley, there is, as already 
mentioned, obviously a zonal connexion. The locality of Locana 
is also remarkable for its considerable outcrop of greyish-green 
lherzolite—a bank 300 metres in depth—which rock Baretti! found 
to be composed of 75 per cent of olivine, and 25 per cent of greyish- 
yellow enstatite and green diopside. 
As previously mentioned, the Ivrea diorite belt reaches north-east 
to the upper Sesia Valley above Varallo, where it becomes schistose 
and gives place to the so-called Sesia gneiss which, further east, in 
the Strona Valley north-west of Lake Orta, takes the name of Strona 
gneiss. Both gneisses are of the kinzigitic type already described, 
ihe Sesia variety being rich, the Strona “variety poor in mica. The 
apparently abrupt cut-off of the diorite in Val Sesia was formerly 
regarded as evidence of a great hiatus due to a fracture fault; in 
reality, however, the diorite has its sequel between the Sermenza 
and Mastellone Valleys north and north-west of Varallo, where the 
1 M. Baretti, ‘‘ Studi Gruppo Gran Paradiso’’?: Mem. Acc. Lincei, Torino, 
1876, p. 195 et seq. 
